Working with lower resolution in compositor viewer?
Scale Node

Something I do is put a 'Scale' node (Shift+A > Distort> Scale) just after any high-res image I want to reduce, setting the scale type to 'Render Size'. This will mean that any inputs will be scaled to your output resolution, set in the Render panel.

Using this method you can then reduce the render size to 50% using the resolution percentage property in the render settings and the compositor will reflect that change (though the compositor may only update to the new resolution when the frame is recalculated).

Here, I have the scale node after a 4k image, but with the resolution percentage set to 50% I only do the compositing with a 2k image, meaning the blur node is processing less pixels and will be faster.





When you want to do the full render you can just disable the scale node with M, which 'Mutes' the node or set the resolution back to full. Other scale node types will also work, instead of 'Render Size', this is just my preferred method.

Note: It is important to put the Scale node directly after the input image/movie so all the compositing is done on the low resolution image. Having the scale node at the end of the node tree would have no effect as the scaling would be happening after all the intensive compositing of the node network is computed.

Compositor Quality

You can also set the 'Edit quality' of the compositor to 'Low' in the Performance area of the Properties menu (N) in the compositor.